Quantum Dots Used to Build RGB Laser

By Pradeep Chandrasekaran on 05/01/2012 11:47 PDT

Lasers produced what can be called almost monochromatic light which means it comes in just one color. By combining 3 single colored lasers, red, green and blue (RGB), it is possible to replicate the whole visible-light spectrum. However, to accomplish this, current technology requires three separate lasers of different color. Now however, a physicist from Brown University called Cuong Dang along with his team of researchers have managed to produce a single material that is capable of producing several different wavelengths and therefore different colors of laser light.

The innovation in essence is a full RGB laser and is constructed with nanoparticles called colloidal quantum dots (CQDs). CQDs are thin films that produce light via quantum excitations but prior to the discovery; researchers found that the energy input required to begin light emission created heat rather than laser light. The size of the quantum dot determines what color laser can be achieved so by overlapping many small patches of CQDs on a surface the researchers observed a spectrum-wide choice of colored laser beams.

Quantum dots are tiny wafers of semi conducting material and their minute size means that quantum excitations are easily controllable. In essence this means that the light emitted by a quantum dot depends not only on the material it is made of but also the physical size of the dot with larger ones emitting longer wavelengths which result in a redder light and smaller sized dots resulting in a more violet shade.

To tackle the issue of the device creating heat rather than laser light, Dang coated the base of the CQDs which are usually made from a material called cadmium-selenium (CdSe) alloy with a second alloy that is not properly described in the papers. With the introduction of the second alloy, the electronic properties were altered and this resulted in light being created as the primary output rather than heat. The team produced three different sized CQDs corresponding to RGB colors as a suspension in liquid after which it was spread onto some glass. After some time and evaporation the resultant product was a film that contained many densely packed quantum dots.

To stimulate light emission, the researchers directed short pulses of laser onto the film which resulted in three different CQDs re-emitting the red, green and blue light wavelengths successfully. After applying a filter to remove most of the light from the original laser source, only the CQDs produced was left. With the final set up, a test confirmed that the usual excitations that led to heat were not present in the quantum dots and to make sure that the device would work in normal conditions, Dang chose to conduct the experiment in air at room temperature as opposed to testing out the device in a sealed vacuum or inert gas.

While this new development is not practical by itself for commercial use since it is just a laboratory prototype, it still has some refining to go through before it can make its way into devices like your Blue-ray player. Nevertheless, it is still a significant advance toward broad-spectrum lasers fabricated from a single material as opposed to using 3 separate lasers to achieve any color in the visible-light spectrum of colors.